This application relates to a detection circuit, and more specifically, to a detection circuit mounted on a power supply system.
An electronic device includes a charging circuit for charging a rechargeable battery serving as a drive power supply. Japanese Patent Publication No. 3428955 discloses a charging circuit for charging a rechargeable battery with charging current supplied from an external power supply. The operation of the charging circuit will now be described with reference to FIG. 1.
A charging circuit 11, which in incorporated in an electronic device, is supplied with DC adapter voltage VAC from an input power adapter 12 coupled to the electronic device. The charging circuit 11 is a DC/DC converter, which generates output voltage Vout from the adapter voltage VAC and controls the output voltage Vout based on a detection value of an output current lout or the like. Specifically, the charging circuit 11 includes a current amplifier 13a, which is coupled to the two ends of a resistor R1 to detect the output current Iout, and a current amplifier 13b, which is coupled to the two ends of a resistor R2 to detect charging current Ichg supplied to a battery BT. The current amplifiers 13a and 13b are coupled to error amplifiers 14a and 14b, respectively. The terminal voltage of the battery BT is supplied to an error amplifier 14c, and the voltage between the terminals of the resistor R1 is supplied to a multiplier 15. The multiplier 15 is coupled to an error amplifier 14d. The multiplier 15 detects the total power amount supplied from the input power adapter 12 based on the current flowing to the resistor R1 and the terminal voltage (i.e., adapter voltage VAC) of the resistor R1. Then, the multiplier 15 outputs a voltage PWRO proportional to the power amount. The error amplifiers 14a to 14d generate control current Isc based on the output current Iout flowing to the resistor R1, the charging current Ichg flowing to the resistor R2, the terminal voltage of the battery BT, and the total power amount (PWRO). A pulse width modulator (PWM) 17 changes the duty cycle for activating and inactivating transistors T1 and T2 based on the control current Isc. The output power of the charging circuit 11 is controlled in accordance with the duty cycle. The battery BT is charged by the charging current Ichg when power is supplied to a system circuit 19 from the charging circuit 11 via a system DC/DC converter 18.
There have been recent demands for controlling the AC adapter to control the output voltage. In this case, the input power adapter 12 shown in FIG. 1 internally incorporates the PWM 17 and the transistors T1 and T2 (e.g., circuit portion depicted in box A drawn by double-dashed line A in FIG. 1), which are controlled by the PWM 17. In such a configuration, the control current Isc generated by the amplifiers 13a, 13b, 14a, 14b, and 14c and the multiplier 15 is supplied from the electronic device to the input power adapter 12. The output voltage of the input power adapter 12 is thus controlled by the control current Isc.
Power information is provided from the input power adapter 12 to the electronic device when the output power of the input power adapter 12 is controlled. The power information is provided as a power limiting signal PWRM or the like from the PWM 17, which is incorporated in the input power adapter 12, to the error amplifier 14d of the electronic device via a cable. The error amplifier 14d amplifies the difference between a power detection signal PWRO output from the multiplier 15 and the power limiting signal PWRM to generate an error voltage. However, the cable includes a resistance component (parasitic resistance). Thus, the power limiting signal PWRM provided to the error amplifier 14d is offset relative to ground level due to the parasitic resistance of the cable. Specifically, the voltage of the power information provided to the error amplifier is lowered by the offset amount. Therefore, the power information is erroneously transmitted, and the circuits in the electronic device generate the control current Isc based on the erroneous power information. As a result, an error voltage deviated from the power supplied from the input power adapter 12 is output from the error amplifier 14d. That is, the generated control current Isc contains a deviation. Accordingly, the required adapter voltage VAC may not be accurately supplied to the electronic device 31.